This invention relates to a process for continuously casting thin slabs, and in particular to a process in which the pulling speed of a solidified slab and the pouring level of a melt in a casting mold are promptly and automatically adjusted to predetermined values during continuous casting through a twin-belt-type continuous casting machine.
Recently, the application of continuous casting to the manufacturing of thin slabs has become widespread. Continuous casting processes have met difficulties including the occurrence of unstable casting conditions, e.g., fluctuations in the molten metal level in a continuous casting mold. This is because the cross section of a thin slab is very small, and even a small fluctuation in casting conditions results in a large variation in the level of molten metal poured into a continuous casting mold.
Furthermore, since an increase in productivity is also desired to lower manufacturing costs, high speed casting is required. Therefore, substantial fluctuations in manufacturing conditions, including the molten metal level in a casting mold, are inevitable.
A prior art process includes the provision of a large-sized tundish positioned below the ladle, a sliding nozzle installed under the bottom of the large-sized tundish, a small-sized tundish postioned below the large-sized tundish, and a continuous casting machine of the twin-belt type (hereunder referred to as a "caster"). The process comprises the steps of pouring a melt first from the ladle to the large-sized tundish and then into the small-sized tundish through the sliding nozzle, over-flowing the melt from the small-sized tundish, casting the over-flowed melt into a moving-belt mold of a caster of the twin-belt type, and solidifying the poured melt in the moving-belt mold.
When the melt accumulates in the tundish to overflow, and the starting of pouring of the melt into the caster is visually detected, the operator starts the caster. The operator then gradually increases the pulling speed of the cast slab to a level previously determined in response to the degree of opening of the sliding nozzle at the beginning of pouring the melt into the caster or the pouring rate of the melt into the caster. When the operator notices through a melt-level-monitoring device that the level of the melt has reached a set point of the level of the melt, the operator then stops increasing the operating speed of the caster and maintains the pulling rate of the slab at a constant level. After that, the operator can manually adjust the degree of opening of a sliding nozzle or the pulling rate of the cast slabs so as to keep the level of the molten metal in the mold constant.
However, during the practice of these processes, the interrelation between the pouring rate of the melt into the caster and the degree of opening of the sliding nozzle varies depending on fluctuations or variations of process conditions such as decrease in the diameter of the opening of the sliding nozzle due to the deposition of the melt onto the nozzle opening. Thus, it is quite difficult to determine an interrelation between them definitely.
Therefore, it is frequently experienced that there is a big difference between the real data and the target value of the flowing rate. In these cases, it is usually impossible to make the pulling speed of the caster as well as the molten metal level in the moving-belt mold reach the target ones in one step. A series of frequent adjustments such as mentioned above, must be repeated in order that the real data may approach the target values. In addition, it will take a long period of time to achieve stable conditions for casting. Therefore, the yield as well as the quality of the resulting slabs are not satisfactory. In addition, sometimes troubles occur which might stop the operations, such as a break-out of the caster, over-flow of the melt from the caster, etc.
In addition, since the operation conditions of a continuous casting process using the twin-belt type caster are not stable, it is necessary to start up the process carefully. Usually it takes a long period of time before the operation is carried out under stable casting conditions. Sometimes, as already mentioned, a substantial fluctuation in the level of the molten metal poured into the mold is inevitable for a twin-belt type caster.